This invention relates to nematic liquid crystal compositions, devices using liquid crystals for information handling and display and to synthesis of liquid crystals.
More particularly, this invention relates to liquid crystal compositions in which the dominant constituent is one or more positive dielectric anisotropy nematic liquid crystals having the general formula ##STR1## wherein R is alkyl or alkoxy having from 1 to 9 carbon atoms.
References Nos. 1 through 14, listed at the end of the specification provide a background of the general subject matter of the present patent and References Nos. 15 through 22 relate more particularly to the compounds discussed herein. The content and disclosure of these references is incorporated herein by reference and a knowledge of the disclosures of these references is assumed. The operation and construction of twisted nematic liquid crystal display units of the type referred to hereinafter are described in these references.
Biphenyl based nematic liquid crystals, "nematogens", are taught in the prior art and considerable work has been done by Gray (Reference 19) and his co-workers. Biphenyl nematogens are considerably more stable than Schiff base compounds and can be actuated with lower saturation voltages than Schiff base. The disadvantageous temperature characteristics of certain of the biphenyl nematogens can be overcome by mixing alkyl and/or alkoxy cyanobiphenyls having differing alkyl and/or alkoxy substituents and low threshold voltages, V.sub.th, and saturation voltages, V.sub.sat, characterize these compositions. Gray et al (Reference 19) report, for example, that for a particularly attractive member of this class of compounds, 4'-n-pentyl-4-cyanobiphenyl, "PCBP" V.sub.th was 1.1 V.sub.rms and an applied voltage of 3 V.sub.rms the compound gave acceptable decay and rise times of 150 and 100 milliseconds, "ms".
Lack of general availability of high quality alkyl and alkoxy cyanobiphenyl nematic liquid crystal compounds, the extremely high cost of such compunds, and the poor nematic temperature ranges exhibited by many of these class of compounds suggested the desirability of alternative compositions which would retain the advantages of biphenyl nematogens and obviate the principal disadvantges of these materials.
One important feature of this invention is the provision of compositions which to a large extent retain the advantages of biphenyl nematogens but reduce the cost and make production of display devices which are actuated with low voltages, i.e. 3.5 Vrms or less, technically, as well as economically, feasible.
Cholesteric liquid crystals have been studied in detail but have not found practical application in information display or handling devices of the type here being considered. It has been suggested (Reference 15) the presence of a small amount of optically active material may preferentially induce one of two equally probable twists in twisted nematic devices and reverse twist induced patches and certain problems related to electric field induced patches are eliminated by the addition of cholesteric compounds to nematic liquid crystal compositions (References 16 and 17.)
One of the features of this invention is the discovery that the decay time of a twisted nematic liquid crystal display device, when actuated with voltges above Vsat, can be significantly improved by the addition of trace amounts of certain optically active liquid crystal constituents.